1. Field of the Invention
This invention relates generally, to spectroscopic based optical fiber sensors. Particularly, this invention relates to scattering, absorption, colorimetric, fluorescent and phosphorescent based sensors.
2. Description of Prior Art and Other Considerations
Spectroscopic based optical fiber sensors can be used in several applications to detect strain, pressure, temperature, chemical species, turbidity, color and other measurands. Two separate approaches have been used with these types of sensors: the optrode (or optode) and the distributed sensing approach.
Optrodes are the simplest type of optical fiber sensors. An optrode comprises an indicator located at the distal end of the optical fiber and an excitation light source and a detector at the proximal end. The excitation light travels through the fiber and interacts with the indicator, producing a spectral signal (fluorescence, phosphorescence, colorimetric and/or absorption based signal). The signal travels back to the proximal end, is collected by a detector and is correlated with the parameter that is being measured. In this case, the fiber has a single sensitive region at its distal end and serves only as a conduit for the light, which propagates undisturbed from the proximal fiber end to the indicator and back.
In the distributed sensing approach, the entire fiber, or sections of the fiber, acts both as a conduit for the signal and as a sensor. The fiber can be manufactured either with a single monolithic cladding, made sensitive to the parameter being measured, or made with several sensitive cladding sections separated from one another. Regardless of the approach, these sensitive, or reactant regions, can be probed by an excitation light, resulting in a multipoint, quasi distributed, sensing device. Whereas a distributed sensor requires a single fiber strand to make multiple spatial measurements, an optrode requires several fibers. Therefore, the advantage of distributed sensing is that it can make multiple spatial measurements with a single device.
The sensing points of a distributed optical fiber sensor can be probed in two different ways: either axially or transversely; however, transverse probing is judged herein to be a superior mode of operation.
Axial probing is widely used as a means to probe the sensor fiber. In axial probing, light is injected from one end of the fiber, along its axis, and interacts with the surrounding cladding via its evanescent wave tail. The cladding then absorbs the probing light in the evanescent region producing either an absorption, scattering or luminescent signal that can be detected at either end of the fiber.
This type of excitation with respect to axial probing, however, has important disadvantages. For instance, the interaction between the evanescent wave of the excitation light and the sensitive cladding is very weak, requiring expensive instrumentation to detect the resulting signal, such as a high power source, an expensive detection scheme and/or a very long optical fiber. Additionally, depending on the arrangement, the alignment of the light source (such as a laser) with the fiber axis requires careful handling.
Schwabacher et al., international publication number WO 2001/71316 ('316), entitled “One-dimensional Arrays on Optical Fibers,” (also, U.S. Pat. No. 7,244,572 issued 17 Jul. 2007) demonstrates a linear array of chemosensors arranged along an optical fiber, with each reactant region in the array being sensitive to a chemical species. Each successive reactant region is separated by a substantially inert region, such as cladding. This substantially inert region must have a minimum length, the preferable length being stated as 250 cm. Publication '316 demonstrates both the axial and transverse methods of excitation, with the axial method being the preferred mode.
In the preferred embodiment, publication '316 employs a narrow axial laser pulse to introduce an excitation light to the optical fiber. Each reactant region is separated by a minimum distance along the fiber, with the region between the reactant regions being substantially inert. This relative long inert section is required by the technology utilized by publication '316, to prevent overlap of fluorescent traces from successive reactant regions. An excitation light from a source (such as a laser, diode laser, gas laser, dye laser, solid state laser, LED, etc) is introduced axially to an optical fiber, with the light then being delivered to the reactant regions.
In order to determine which reactant region, among several or even hundreds, is producing a signal, the time delay between the excitation pulse and return signal must be precisely known and correlated with the distance each particular reactant region is from the source. This determination involves the measuring of time, distance, and wavelength by use of precise instruments such as by the use of an oscilloscope and a photomultiplier tube. This arrangement requires an extremely long length of fiber in order to measure hundreds of species, and thus increases the overall size and complexity of the analyzing device. Furthermore, the precision instruments can increase the overall cost of the instrument significantly.
The excitation light can also be introduced to the reactant regions on the sensing fiber by an excitation fiber or fibers. This also requires the axial introduction of light to the excitation fiber. One excitation fiber per reactant region is required in one embodiment, with each fiber introducing the excitation light transversely to the reactant region of the sensing fiber.
Another embodiment requires the use of beam splitters to deliver the excitation light transversely to the reactant regions. The beam splitting technique make use of expensive high power lasers resulting in a decay of the intensities as more beam splitters divert the excitation light to the sensitive coating.
In another scheme, the excitation (or illumination) fiber is prepared by removing its cladding from small sections along its length, with these sections then being installed adjacent to the reactant regions on a nearby sensing fiber, and allowing its evanescent field to transversely excite the sensing fiber. A disadvantage is that the evanescent field of the excitation fiber is very weak, thus delivering very little power to the sensing fiber. Additionally, other methods of axial and transverse excitation are revealed; however, these methods were, on average, not cost effective.
Although these embodiments of publication '316 are assumably operational, they are limited by complexity, manufacturing expense, and robustness of design. In order to manufacture alternating sections of reactant and inert regions, cladding must be removed only in the reactant regions, leaving intact the remaining inert regions. This alternating removal of cladding increases the expense and complexity of mass production, limiting automation options in manufacture.
Additionally, other techniques utilized in industry require the use of expensive instrumentation such as an optical time domain reflectometer (OTDR). Costing on the order of US $3,000 or more, an OTDR adds considerable expense to any system that uses the axial excitation technique. Also, the wavelengths availability of OTDR systems is limited, restricting the choices of reagents that can be used with the sensor. A further disadvantage of present systems is the interference of the signal detected by the OTDR caused by inadvertent bends and physical irregularities in the waveguide material, either of which can vary the fiber's refractive index. Furthermore, present techniques lack refinement of spatial resolution, on the order of approximately 10 cm. A more refined spatial resolution is needed.
While transverse probing of the sensitive region appears to be a superior technique that can produce a substantial signal, the prior art failed to identify this and other additional advantages. Side illumination, when properly done, can probe very small sections of a sensitive fiber, leading to a sensor with a very high spatial resolution and, consequently, multiple sensing points along the fiber length. For example, a high spatial resolution, of 5 mm can lead to ten sensing points for every 5 cm of fiber resulting in a high density sensor array in a single fiber. High spatial resolution also is desired in applications in which there is a strong variation of the temperature and/or concentration of a chemical species along the length of the optical fiber. The monitoring of chloride ions inside concrete structures, serves as an example of where the sensing can be made at discrete narrow locations along the fiber. Previous endeavors failed to provide a simpler illumination technique that leads to a low cost, rugged, distributed sensor. More importantly, the prior art has failed to recognize that a side illuminated optical fiber sensor without a chemical indicator in its cladding can detect certain parameters.
There are many needs and desires to overcome these and other deficiencies and/or problems in the prior art, as exemplified but not necessarily limited to the following:                a. an inexpensive probing light source that can additionally provide a high spatial resolution to the fiber sensor, on the order of 5 mm or less, enabling the pinpointing of the exact location of detection;        b. a cost effective optical fiber sensor system that uses inexpensive, off the shelf, commercially available devices that can be fabricated by automated means;        c. a flexible device that can be used throughout the infrared, visible, and ultraviolet regions of the electromagnetic spectrum;        d. a rugged sensing device that can be easily aligned and is not affected by outside interference such as bending and ambient light;        e. a generic design that can be adapted to monitor different measurands is needed;        f. an intense, yet, cost effective probing light source for a fluorescent based and absorption based fiber that can produce a strong signal that can be easily detected;        g. a modular sensing system design that can be easily updated with the evolving technology; and        h. a sensing system that does not require a chemical indicator immobilized over the surface of the fiber to detect a given measurand.        